The XT isn't a huge upgrade over the previous Rebel, but for what I do it's worth trading up.
The non-L lenses are mostly pretty much the same as far as image quality; some of them are slightly better than others but not even close to most of the L stuff. This is speaking from my experience - others may have gotten a great non-L lens and a crappy L lens (quality control isn't 100.0%) and compared the 2 with different results.

All of the L stuff I've owned has been prime lens quality.
If you're not going for the best quality 20x30 enlargements, I'd just recommend one of the 75-300 consumer lenses. There may be slight differences between the IS version & non-IS, but they're seriously all good enough for professionals. And note that with IS, it will freak out if the subject is moving.

Oh, and it uses massive amounts of battery power.
